Colorectal cancer (CRC) has served as a unique platform for durable insights into and advances in molecular pathogenesis, chemoprevention, diagnosis, and therapy, not only in this venue but expanded into other disease states. The elucidation of these comprehensive aspects of CRC has been facilitated through the identification of key genes and molecular pathways. It is well accepted that the majority of sporadic colorectal cancers feature the chromosomal instability pathway, involving alterations in key tumor suppressor genes (APC and p53, but also SMAD4) and oncogenes (especially Ras, but also EGFR, c-myc, B-Raf) and their downstream effectors. A subset of sporadic colon cancers undergo microsatellite instability (MSI) pathway, typically right-sided with B-Raf mutations but without Ras mutations. Recently, it has been advocated that certain colorectal cancers feature hypermethylation. We have genetically and physically mapped a region of loss of heterozygosity (LOH) on chromosome 22q13.31 as a basis to understand the role of new genes in colorectal carcinogenesis (and other cancers). We now have discovered a new telomeric region of LOH, which harbors two microRNAs, namely Let-7a3 and Let-7b, and these microRNAs are downregulated in up to 40% of colorectal cancers. MicroRNAs have become increasingly recognized for their pivotal, diverse roles in development, differentiation, proliferation, and cancer, which is achieved the degradation or interference with translation with messenger RNAs. We hypothesize that the Let-7a3 and Let-7b microRNAs are instrumental in colorectal cancer progression, and this may be achieved by targeting the Ras oncogene, and also, the c-myc oncogene. This novel hypothesis will be pursued by the following interrelated Specific Aims: (1). To identify the roles of the Let-7a3 and Let-7b microRNAs in colorectal cancer. A. To determine the relationship between these microRNAs and clinical parameters and molecular parameters; B. To determine the function(s) of Let7a and Let7b in colorectal carcinogenesis through genetic /overexpression/knockdown studies of these microRNAs with determination of effects upon cellular proliferation, apoptosis, migration and invasion, which are critical features of the colon cancer initiation and progression. (2). To determine if the Ras and c-myc oncogenes represent targets of Let-7a3 and Let- 7b. This will be achieved through examination of Ras transcriptional and protein activities in the setting of genetic approaches, namely Let-7a3 and Let-7b overexpression/knockdown. (3) To determine the functional consequences of restoration and inhibition of Let-7a3 and Let-7b in colon cancer cells in vivo. This will be assessed by innovative three-dimensional organotypic cultures, which mimic the colon cancer microenvironment, and in athymic, irradiated nude mice through bioluminescence of tumor growth. In aggregate, our studies have the means to unravel new pathways of regulation of the Ras and c-myc oncogenes by the Let-7a3 and Let-7b microRNAs in colon cancer, especially where sporadic colorectal cancers (chromosomal instability pathway) do not harbor Ras mutations. PUBLIC HEALTH RELEVANCE: Colorectal cancer (CRC) serves as a paradigm for the investigation of basic mechanisms and translation into novel chemopreventive, diagnostic and therapeutic strategies. The underlying pathogenesis of CRC may involve the Let-7a3 and Let-7b microRNAs, which in general are critical regulators of normal cellular processes to malignant transformation. Our studies hope to reveal new paradigms in CRC pathogenesis, and offer opportunities for exploiting this knowledge in novel diagnostics and therapeutics.